A floating flap gate exists for which there is no delay in a floating action of a door body during an initial influx of water, so there is no overflow of water onto land during an initial influx of seawater at a time of a tsunami or a high tide, and which does not exhibit a hazardous behavior such as suddenly falling (e.g., Patent Reference 1).
A prior art floating flap gate 101 shown in FIGS. 5A, 5B and 5C has a single rod 103 attached, for example, across an entire width-wise direction of a forward end portion of a door body 102, functioning to support a load resulting from a water pressure and to attach one end of a wire rope 104.
The other end of the wire rope 104 is attached to a counterweight 120 via a fixed pulley 106 mounted on a door bumper 105 above a forward end portion 102b of the door body 102, and a fixed pulley 107 mounted above a base end portion 102a of the door body 102. Accordingly, during ordinary operation, the weight of the counterweight 120 is constantly acting on the forward end portion of the door body 102. In FIGS. 5A, 5B and 5C, rs is a channel surface at an opening, and 121 is a door bumper (holding portion).
At the initial stage of seawater influx, the counterweight 120 of the prior art floating flap gate 101 drops, so that the door body 120 is pulled in a rising direction, helping it to rise (see the operation illustrated in FIGS. 6A-6B).
In this type of floating flat gate, which employs a tension of a rope at both ends of a forward end portion of a door body, the greater the width of the door body, the greater the bending of the forward end portion of the door body resulting from the rope tension during ordinary use. There is a risk that a bending of the forward end portion of the door body could result in a rising upwards from the channel surface, which would inhibit safe travel of people and vehicles.
Increasing the thickness and rigidity of the door body was considered as a means to keep the amount of bending below an allowed value, but this inevitably increased the weight of the door body. An increase in the weight of the door body results in greater weight of the equipment as a whole, and thus increasing the cost.